1. Field of the Invention
The present invention relates generally to a Frequency Division Multiple Access (FDMA) system, and in particular, to a method and apparatus for setting transmission power of a data channel in an FDMA system.
2. Description of the Related Art
The uplink multiple access scheme used in mobile communication systems is roughly divided into a non-orthogonal multiple access scheme and an orthogonal multiple access scheme. The ‘non-orthogonal multiple access scheme’ refers to a multiple access scheme in which uplink signals transmitted from multiple mobile stations are not orthogonal with each other. The Code Division Multiple Access (CDMA) scheme is one example of the non-orthogonal multiple access scheme. The ‘orthogonal multiple access scheme’ refers to a multiple access scheme in which uplink signals transmitted from multiple mobile stations are orthogonal with each other. The Frequency Division Multiple Access (FDMA) scheme and the Time Division Multiple Access (TDMA) are examples of the orthogonal multiple access scheme.
In the general packet data communication system, a mixed scheme of the FDMA scheme and the TDMA scheme is used as an orthogonal multiple access scheme. Transmission signals of several users can be distinguished on the basis of frequency and time. In the present application, the ‘FDMA scheme’ refers to the mixed scheme of the FDMA scheme and the TDMA scheme.
Typical examples of the FDMA scheme include an Orthogonal Frequency Division Multiple Access (OFDMA) scheme and a Single Carrier Frequency Division Multiple Access (SC-FDMA) scheme. The FDMA schemes refer to a multiple access scheme for allowing multiple mobile stations to transmit signals over different subcarriers, making it possible to distinguish the signals of the mobile stations.
With reference to FIG. 1, a detailed description will now be made of a transmitter employing an Interleaved Frequency Division Multiple Access (IFDMA), also known as a Distributed FDMA (DFDMA) scheme, which is one of the SC-FDMA schemes.
FIG. 1 illustrates an exemplary structure of an IFDMA transmitter. Although the IFDMA transmitter is realized by utilizing a Fast Fourier Transform (FFT) and an Inverse Fast Fourier Transform (IFFT) in FIG. 1, the transmitter can be realized in another manner. The FFT/IFFT-based realization method shown in FIG. 1 can facilitate a change in IFDMA system parameters because of its low hardware complexity.
Referring to FIG. 1, in the IFDMA transmitter, an FFT 104 is disposed in front of an IFFT 106 used for multi-carrier transmission. In FIG. 1, therefore, transmission symbols 100 are input to the FFT 104 on a block-by-block basis. The signals output from the FFT 104 are input to the IFFT 106 at regular intervals, and IFDMA transmission signal components are transmitted over regular-interval subcarriers in the frequency domain. Generally, a size N of input/output signals of the IFFT 106 is greater than a size M of input/output signals of the FFT 104.
In the OFDM transmitter, as the transmission symbols 100 are directly input to the IFFT 106 without passing through the FFT 104 and then transmitted over several subcarriers, a high Peak-to-Average Power Ratio (PAPR) occurs. Even in the IFDMA transmitter, the transmission symbols 100 are finally processed in the IFFT 106 and then transmitted over multiple carriers. However, as the transmission symbols 100 are pre-processed by the FFT 104 before undergoing the final processing, the IFDMA transmitter can obtain an effect similar to the effect that the output signals of the IFFT 106 are transmitted over a single carrier, by means of the cancellation function between the FFT and the IFFT, thereby achieving a lower PAPR compared to the OFDM transmitter.
Finally, the IFDMA transmitter converts the outputs of the IFFT 106 into a serial stream in a Parallel-to-Serial Converter (PSC) 102, and adds a Cyclic Prefix (CP) thereto by means of a CP adder 108 before transmission like in the OFDM system, thereby preventing interference between multipath channel signal components.
FIG. 2 illustrates an exemplary structure of a Localized Frequency Division Multiple Access (LFDMA) transmitter, similar to the IFDMA transmitter, capable of ensuring the orthogonality between multiple access users and achieving a lower PAPR compared to the OFDM transmitter, based on single-carrier transmission.
As can be seen in FIG. 2, the difference between the LFDMA scheme and the IFDMA scheme in terms of the transmitter structure is in that outputs of an FFT 204 are applied to inputs of an IFFT 206, which have serial indexes. Therefore, in the frequency domain, LFDMA transmission signals occupy the band composed of adjacent subcarriers used when the outputs of the FFT 204 are mapped to the inputs of the IFFT 206. In other words, in the frequency domain, the IFDMA transmission signals occupy subcarrier bands (or subbands) scattered at regular intervals, and the LFDMA transmission signals occupy the subcarrier band composed of adjacent subcarriers.
In the general uplink mobile communication system, a base station can support a higher system capacity with the limited wireless resources by channel selective scheduling. The term ‘uplink’ as used herein refers to transmission from a mobile station to a base station, and the term ‘channel selective scheduling’ as used herein refers to a technology for allocating, to a mobile station, the time interval or frequency interval having the better channel environment for the channel that varies in the time axis or the frequency axis, to allow the mobile station to transmit data, thereby achieving system capacity improvement.
FIG. 3 illustrates exemplary scheduling on the time axis.
Referring to FIG. 3, in step 303, a mobile station 302 transmits a pilot signal to undergo scheduling for data transmission. A base station 301, as it can detect a channel status of the mobile station 302 depending on the pilot signal transmitted from the mobile station 302, can determine whether to perform scheduling and select an appropriate demodulation scheme and code rate when the mobile station 302 determines to perform scheduling. In step 304, the mobile station 302 sends a status report. The term ‘status’ as used herein refers to a status of the buffer, data in which the mobile station desires to transmit, or a status of the power at which the mobile station desires to transmit data. The mobile station 302 can report the amount of packet data or a service priority of packet data, as the information indicating the buffer status, and can report the maximum transmission power as the information indicating the power status. In step 305, the base station 301 performs a scheduling procedure on the mobile station 302 based on the information acquired from the status report and the pilot signal. After the scheduling, the base station 301 transmits grant information to the corresponding mobile station 302 in step 306. Upon receipt of the grant information, the mobile station 302 transmits data in response thereto in step 307.
In the general scheduling scheme, the power information can be either transmitted or not transmitted to the base station by the mobile station according to the algorithm for setting transmission power. In other words, in the system using a closed-loop power control scheme, the base station cannot determine the maximum data rate allowable to the mobile station until the mobile station transmits power headroom information to the base station. However, in the system using an open-loop power control scheme, even though the mobile station transmits no power information, the base station can determine the maximum data rate allowable to the mobile station depending only on the Signal-to-Interference Ratio (SIR) of the pilot received from the mobile station.
Next, a description will be made of a method for setting power when the mobile station transmits data over a Shared Data Channel (SDCH). For data transmission power, the conventional Wideband Code Division Multiple Access (WCDMA) system uses different beta values according to desired transport formats. The term ‘transport format’ as used herein refers to a value that varies according to the size of the desired transmission data, or the service type of the desired transmission data. The beta value indicates the ration in which the system will transmit data in proportion to the transmission power of the pilot channel controlled by closed-loop power control. The beta value can be signaled, or can be set by a calculation.
WCDMA allows the mobile station to freely determine a transport format according to the amount or type of its desired transmission data and to efficiently set the uplink transmission power according to the transport format. However, in the current FDMA system, discussion is being conducted on a transmission power setting method which is different from the foregoing method, for the following reason. Because the FDMA system, unlike the WCDMA system, uses orthogonal frequency resources, there is no interference between mobile stations in the cell, and in the scheduling method now under discussion, the mobile station cannot select the amount and Modulation and Coding Scheme (MCS) level of frequency resources corresponding to the transport format, and simply transmits the data according to the information set by the base station. However, even in the OFDM system, because the interference to adjacent cells remains unchanged, the OFDM system can set transmission power considering only the interference level.
A detailed description will now be made of a method for setting transmission power of the data channel. A mobile station transmits a reference channel signal for closed-loop power control, and adjusts transmission power of the corresponding channel in response to a power control command received from a base station. Here, the transmission power of the reference channel is referred to as ‘P_ref’ and the transmission power of the data channel is referred to as ‘P_data’. P_data can be set using Equation (I).P_data=P_ref−10 log 10(Nc_ref)+10 log 10(Nc_data)+Offset_data  (1)
In Equation (1), Nc_ref and Nc_data denote the number of frequency subcarriers used for the reference channel and the number of frequency subcarriers used for the data channel, respectively, and Offset_data denotes a power ratio per frequency tone of the reference channel to the data channel, and is referred to as a ‘power offset value’. As described above, Offset_data is a value that the base station sets regardless of a data rate of the desired transmission data, or a value that the mobile station controls depending on such factors as the interference to neighbor cells.
A description will now be made of an operation performed when the mobile station desires to transmit data at a data rate less than the scheduled data rate. In the FDMA system, because the base station has correct information on the amount of data that the mobile station desires to transmit, depending on a buffer status/power status report, the mobile station basically transmits the data as scheduled by the base station. However, when the status report is delayed or the status report is not correctly made due to the limited amount of information, and when resources are allocated to be occupied for a predetermined time in a Voice over Internet Protocol (VoIP) scheduling process, the base station may not have correct information on the buffer status of the mobile station. When the mobile station needs a data rate greater than the data rate allocated by the base station, the mobile station can increase the data rate by sending a scheduling request. However, if the mobile station has no right to select the data rate, the mobile station has no way but to adjust the data rate to the data rate format in which the upper layer allocates the total amount of data by zero padding. This method is simple, but may cause unnecessarily interference. Even though the mobile station can use low transmission power according to the low data rate, the zero-padded data is transmitted at a high data rate, so the mobile station may use high transmission power, causing interference. Therefore, as a method for addressing this problem, discussion is being conducted on the method in which if the base station schedules particular frequency resource and MCS level, the mobile station selects a data rate from among several limited data rates based on the scheduling result, and transmits data at the selected data rate.
After the base station has allocated frequency resources for data transmission, when the mobile station simultaneously transmits data and control information as it has control information to transmit over an uplink, the mobile station may occasionally use some of the control information allocated for the data transmission, for transmission of the control information. In this case, the mobile station performs puncturing on some bits of the data, and due to the puncturing, the data is transmitted with a small number of physical layer bits, so it may have a code rate higher than the expected code rate, causing a decrease in the data transmission quality. For the case where the data and the control information are simply simultaneously transmitted, the mobile station can transmit the data and the control information with the power increased by a predetermined level. However, for the case where the amount of control information is variable, for example, for the case where the mobile station transmits Acknowledgement (ACK)/Negative Acknowledgement (NACK), when the mobile station transmits Channel Quality Indication (CQI) or transmits both ACK/NACK and CQI, the amount of resources that should undergo puncturing changes. Therefore, it is not preferable to increase the power as the determined power level.
No discussion has been made on the method for setting transmission power in transmitting data at the low data rate. However, because the Offset_data calculated by Equation (1) is not defined as a value determined depending on the data rate, if the mobile station uses the intact Offset_data calculated by Equation (1), it may not have the merit of transmitting data at the low data rate.